Pilot operated pressure reducing valves are well known. See for instance page 123 of the book "The Hydraulic Trainer" of Mannesmann Rexroth GmbH. Thus, it is known to branch off the pilot fluid or liquid for the pilot valve, at the output of the pressure reducing valve i.e. outside the effective area of the main or control spool. Alternatively, pilot fluid can be branched off in the area of the control egdes of the control spool, and in each case the control spool comprises a centrally located port acting as a throttle means. Via the throttle means the pilot fluid is directly supplied to the control space or chamber of the control spool. If the pilot fluid is taken from the output of the pressure reducing valve, it is necessary to limit the volume flow or stream as well as the pressure differential to values which are below the customary limits for the respective nominal sizes of the valve. Otherwise a "closing effect" of the main valve will occur, a situation which makes the main valve ineffective.
When the pilot fluid in the control range (or area) of the control spool decreases, the control volume flow changes in relatively large areas or ranges depending on the main volume flow and the pressure differentials between the inlet B and the outlet A. The tendency of change is decreasing up to about the middle of the volume flow range, and beyond, it is increasing. The decreasing tendency of change is increased with an increasing pressure differential (B A). In the falling range of the pilot volume flow, instabilities can occur and for a high pressure differential (B A) the tendency of "valve closing" occurs. In the increasing branch of volume flow the valve tends to whistle (pilot control whistling). To avoid the occurences of instabilities it is neseccery to select the input and dampening throttle (or orifice) to be relatively small. This, however, has a negative influence on the dynamic response of the valve.
It is an object of the present invention to provide a pilot operated pressure reducing valve such that it shows, in comparison with other pressure valves, good characteristics for control and stability in the entire area or range of the volume flow and pressures.
Generally, the present invention is directed to overcome one or more of the problems of the prior art.